In the melt-spinning process, a stream of molten material is divided into a plurality of filaments, cooled below their solidification point to form the desired product. It is preferable that cooling be effected to a point below the glass transition temperature as well. Once this has been accomplished, the filaments are drawn off and wound in a conventional manner. In order to produce a product of high quality, it is essential that the melt be as homogeneous as possible and the cooling conditons be uniform.
In addition, the homogeneity of the melt is adversely affected by thermal decomposition. There should be no zones in which the melt throughput is slow or stagnant, as these will cause clogging and breakage of filaments. This can be best accomplished by the use of round nozzles, having a plurality of openings therein.
However, these nozzles possess certain disadvantages with regard to cooling of the filaments produced thereby. Often, this has been done by blowing a transverse stream of air across the filaments. In order to accommodate this, it is necessary that the nozzle diameter be very large and the number of openings per plate similarly be quite low. Moreover, the filaments on the near side of the transverse stream are cooled more rapidly and to a greater extent than those on the opposite side. When the number of openings and the throughput thereof is increased, this difference is amplified. This will have an adverse affect on such properties as the uniformity of stretch behaviour, elongation at break, shrinkage, and coloration.
One "solution" to the foregoing is the provision of rectangular nozzles having 2,000 to 3,000 openings therein. These would replace the round nozzles which would have 600 to 800 openings at most. However, rectangular nozzles, because of their shape, have a greater tendency to block the melt stream than do round nozzles. Obviously, those openings near the corners would have a lower throughput than those in the center. This variation is undesirable and, for this reason, rectangular nozzles must be changed far more often than round ones.
Still another approach is to use circular nozzles which are provided with a very large number of radially symmetrical openings. The air stream is not introduced transversely, but rather radially from all sides. U.S. Pat. No. 3,299,469 describes such a process.
However, this, too, presents serious problems. When the air blows inwardly, it tends to compact the filaments, reducing the space between them. In some cases, the filaments actually touch one another and, because they are not yet cool, fusion takes place. On the other hand, if the coolant stream is moving outwardly, the filaments are blown away from one another and there is little or no tendency for them to compact.
Furthermore, when blowing inwardly, the air is heated as it moves to the center of the bundle of fibers. Hence, at that point its effect is sustantially reduced. However, if the flow is in the opposite direction, the coolest air is introduced at the center and warms up as it reaches the periphery of the filaments. However, at this point, the outside air can assist in cooling the material. Thus, the ambient air is useful at the place it is most needed.
Such patents as U.S. Pat. No. 3,858,386; 3,969,462; 4,285,646; and EP Nos. 40,482; and 50,483 broadly teach blowing from the center outwards. However, introduction of the air stream is extremely difficult in such a situation and is undoubtedly the reason that this process has found little acceptance.
If the air stream of the foregoing type is introduced below and flows upwards, the stream crosses the filament path. It is necessary, when using such a device, to divide the exiting filaments into two bundles moving side-by-side. In this way, the freshly-spun filaments are not disturbed by the air stream inlet pipe. Such an process is described in U.S. Pat. No. 4,285,646 (Column 2, line 6 to 68). There are a number of disadvantages to this process. Great difficulties arise when it is necessary to start up the operation after interruptions resulting from, for example, filament breakage, nozzle change, cleaning, etc. The reference makes no mention of dealing with these problems. The fibrils which, at this point, are insufficiently strong, but quite tacky, readily adhere to the air outlet. They then break and other fibrils stick to them and also break. This is such a serious problem, that even skilled personnel have the greatest difficulty in properly regulating such a process.
In order to solve the foregoing problems, such patents as U.S. Pat. No. 4,285,646; EP No. 40,482; and EP No. 50,483 teach introducing the air stream from above centrally through a group of nozzles. However, as in the other cases, the solution brings additional problems. The melt in the nozzle should not be cooled by the air stream, as this assists in causing unwanted blockage. Moreover, the air stream should not be heated by hot nozzles. Hence, it is necessary to isolate one from the other. The only way this can be done is to increase the nozzle diameter to a point at which the round nozzle no longer gives a melt flow which is radially symmetrical.